This invention relates to a method of treating and breaking up ("globulating") a liquid with the help of centripetal force, and also apparatus for performing the method.
In this context, the term "liquid" refers to solutions and suspensions in water, organic liquids and molten metal.
A large number of procedures are known for treating and breaking up liquids, in which the treatment can consist of various ways of introducing substances into a liquid, and stirring this in order to make the liquid as homogeneous as possible. The term "treatment" in this context also refers to the removal of substances from a liquid, for example a gas, by airing the liquid. Airing is particularly efficient when the liquid is broken up into small drops and thrown out in a vacuum or in a controlled atmosphere. Treatment can also involve the transfer of solid compound in a volatile reaction, for example, by treating the liquid with chlorine to remove contaminants in the form of chlorides.
This present invention was made in connection with the refining of molten metal, with particular reference to the treatment of molten aluminium. In this connection, the operation usually involves the removal of hydrogen and/or solid particles. It can also often be necessary to reduce the content of unwanted alloying metals, such as magnesium and iron.
Patents have been sought for a number of procedures for refining aluminium.
In Norwegian patent application No. 82 2913, Pechiney have made an extensive analysis of the problems in question and have, as a result of their work, invented a rotating gas-dispersing device which consists of a cylindrical rotor equipped with blades immersed in the metal, and connected to a hollow drive shaft through which the gas can be introduced. The rotor is perforated with paired channels, one of the channels in each pair being for molten metal, and the other for gas.
This apparatus stirs the surface of the metal which is to be treated, and is costly to make and operate. The channels become corroded, and the gas is not used efficiently. Th equipment does not operate satisfactorily.
In their Norwegian provisional patent (utlegningsskrift) No. 137 601, Union Carbide have approached the problem from a different angle. Pre-heated inert gas is introduced into an enclosed refining vessel through a rotating gas injector. According to Union Carbide's Norwegian provisional patent (utlegningsskrift) No. 134,754, this consists of a vertical shaft, with a drive arrangement at its upper end, and at the lower end fixed to a winged rotor. The shaft rotates in a stationary sheath, the lower end of which is equipped with wings and vertical channels between the wings, whereby by rotating the rotor, gas is injected into the molten metal.
This method produces a lively circulation of the gas in the metal and operates efficiently. However, the system is awkward. It is difficult to empty the apparatus, and the rotor may break if solid particles enter the system.
An object of the present invention is to avoid all forms of narrow channels in an apparatus for treating molten metal and other liquids.
The invention refers to a procedure for treating and breaking up a liquid by means of centripetal force. This is achieved in that the liquid is introduced continuously through the bottom of a rotating hollow body immersed in the liquid, and in that the rotating liquid is forced out through holes in the side wall of the hollow body, and the treatment takes place while the metal is rotating.
U.S. Pat. No. 3,753,689 discloses a method of treating a nonhomogeneous melt with a gas. The metal is placed in a container, a ladle, in a reactor. The reactor can rotate rapidly about its vertical axis, and the surface of the metal thus aquires the shape of a paraboloid of revolution, with a significantly larger surface than it had when the ladle was stationary. A suitable gas is introduced down into the ladle, and the metal is thus treated with the gas.
The melt, according to this invention, acquires an enlarged surface for the gas treatment, but it is still only a question of surface treatment.
Norwegian Pat. No. 24 835 describes the introduction of a rotating conical body down into the liquid. On account of the rotation, the liquid rotates, and the centripetal force causes the metal inside the body to rise up along the walls and be thrown out over the edge, falling like fine rain onto the surrounding liquid.
This Norwegian patent claims that the invention provides an efficient way of cleaning, scrubbing, drying and absorbing a gas. The opposite process, i.e. treating a liquid with a gas is, on the other hand, the purpose of the present invention.
Norwegian provisional patent (utlegningsskrift) No. 133 531 discloses an apparatus for producing granulated metal, consisting of a rotating hollow body with a hole in the bottom and holes in the side, partly submerged in the molten metal, and with a vertical hollow drive shaft. When this hollow body is caused to rotate, the metal, on account of the centripetal force, will be forced up inside the hollow body and ejected through the holes in the side. In operation, the holes in the side wall of the body can become clogged. Small quantities of inert gas can be introduced through the hollow shaft, and in this manner the holes in the side wall are kept open. However, the gas added has no chemical function.
To perform the method of the present invention, the rotor must have a drive shaft, suspended over the parabolic surface of the liquid, which holds the rotor at a fixed or variable level with respect to the surface of the liquid during rotation, and the rotor must have one or more holes in the bottom and in the side wall. It is necessary that the holes in the side wall have a diameter of from 1 mm to 50% of the diameter of the rotor. The bottom hole, which does not have to be circular, can have a diameter of 50-100% of the diameter of the rotor.
The number of holes in the side wall and how they are placed will depend upon the operation the apparatus is required to perform. The total area of the side holes may be equal to, or greater, or less than the area of the bottom hole. In certain cases, it can be advantageous to have several bottom holes. In operation, the side holes are below the uppermost level of the parabolic liquid surface inside the rotor yet above the lowermost level of the parabolic liquid surface formed by rotation of the rotor.
Depending upon the nature of the operation, the rotor may, in action, have the holes in the side wall below the surface of the liquid, above the surface of the liquid, or both over and under the surface of the liquid outside the rotor.
The rotor shaft is preferably hollow, and can be used for adding solid, liquid or gaseous material to the liquid.
The rotor must be made of a material which can withstand the centripetal forces and the stresses imposed by the liquid. For treating molten metals, the rotor can be made in one or more of the following materials: graphite, aluminum titanate, boron nitride, alumina, metallic titanium or more conventional ceramic materials.
With expedient choice of materials for making the rotor, unalloyed and alloyed metals, such as aluminum, magnesium, iron and ferro alloys, can be treated by the method in question. Water and an aqueous suspension such as sewage can also be treated according to the invention.